is to add the enclosed statements only when the <MACRO_NAME> was defined before. the definition of a macro may happen either in the preprocessor definitions of the project, or in some code - almost ever in a header file, included before the #ifdef block.

i only found WIN32 in my sources and VC includes.

in the visual studio sdk, i also found _WIN32, in some internal windows headers. generally names with an underline prefix are reserved for internal use of the compiler or sdk. they shouldn't be user defined (although this was widely ignored in my experience).